A review of forest N cycling literature indicates that most forest ecosystems contain less N than would be expected from even modest inputs of N from atmospheric deposition and N fixation over millennial time scales. Periodic fire could account for this disparity, even in humid systems during drought periods. In contrast to the millennial time scale patterns noted above, several forest ecosystems appear to accumulate more N than can be accounted for by measured or estimated inputs over decadal time scales. There appears to be some disparity between results from short term process studies and those from longer term budget analyses. Unmeasured inputs by dry deposition, non-symbiotic N fixation, or (in ecosystems with sedimentary parent materials) weathering of N from rocks may account for this occult N when it occurs. Research over the last two decades has suggested that N retained within forest ecosystems is not leached away after inputs have slowed, but remains within the system unless it is harvested or burned.

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